0001197: small bug in sample wave form display after scrolling when max. zoom is active

Description

Vertical lines occur in the graphical representation of the sample data when on maximum zoom-in, after you scroll back-and-forth rapidly.
The screenshot should speak for itself. The artifacts come and go as soon as they go from off-screen to on-screen.
The bug is not isolated to a particular module. I have not tried to reproduce it on different machines.
I use the 32bit version of MPT 1.28.03.00, on a 64bit Win7 Pro.

Steps To Reproduce

In the "Samples" tab, zoom in as much as possible so you get a detailed view of the sample data (each sample point is now separated about 50 pixels from the next one, the display interpolates linearly).

move the horizontal scroll bar on the bottom back and forth to reproduce the "problem"